Manhattanhenge: Time for Sunset's Rite of Spring on New York Streets

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May 27, 2014 / 10:02 PM GMT / Updated May 29, 2014 / 5:18 PM GMT

The weather forecast may be iffy this week, but that's not going to stop New Yorkers from looking for a rare celestial event: Manhattanhenge, a time when the rays of the setting sun shine right through the concrete canyons in the heart of the city.

Some photographers have already gotten a head start on Manhattanhenge, but only four sunsets per year officially qualify. The first comes at 8:16 p.m. ET Thursday. That's when a pretty half-sun should be visible on the horizon, right between the skyscrapers that line the streets on Manhattan's main street grid. The sun's full disk should sit right atop the horizon on Friday evening at 8:18 p.m. ET.

The two other official dates for Manhattanhenge come on July 18 (full sun at 8:24 p.m. ET) and July 19 (half sun at 8:25 p.m. ET), according to the American Museum of Natural History's reckoning.

Worshiping war and baseball?

Manhattanhenge was first popularized by Neil deGrasse Tyson, who heads up the museum's Hayden Planetarium and is the host of the updated "Cosmos" TV series. Back in 2001, he mused that future civilizations would surely presume Manhattan's street grid was laid out for astronomical purposes, just as we presume that Stonehenge served as an ancient sun observatory. Hence the name Manhattanhenge.

Tyson noted that the dates marked by Manhattan's solar alignment came around Memorial Day and baseball's All-Star break. "Future anthropologists might conclude that, via the sun, the people who called themselves Americans worshiped War and Baseball," he wrote.

Of course, the alignment is merely the result of the main street grid's 29-degree tilt with respect to geographic north. That means that Manhattan's main east-west thoroughfares come into the spotlight as the sun's setting point creeps northward toward the June solstice, and again as that point retreats back southward. The timing of Manhattanhenge has nothing to do with the sun itself, and everything to do with Earth's axial tilt and seasonal procession around the sun.

Wherever you go, arrive early — and be prepared for crowds and/or clouds. The New York City Department of Transportation says it's not planning any traffic closures for this year's Manhattanhenge, but there can be a lot of picture-takers standing in the streets as sunset approaches. The farther east you can get on your chosen cross-street, the more impressive the view.

Not a New Yorker? Different street-grid alignments can provide "henge" experiences in other cities, such as Chicago (around the March and September equinoxes), Philadelphia (in April and September) and Toronto (in February and October). Heck, there's even an "MIT-henge" on the Massachusetts Institute of Technology's campus in January and November.

But this week, the best way for out-of-towners to get the hang of it is to watch for the #manhattanhenge and #manhattanhenge2014 postings to Twitter and Instagram. Want to share your pictures with us and the world? Just add the hashtag #NBCHenge and we'll pass 'em along.